ABSTRACT: To maximize their growth, grazers should shift their behavior to balance the cues from predators and periphyton in the field. As a stream grazer seeks periphyton that are heterogeneously distributed, a shift in its behavior on the basis of the intensity of microalgal cues would be beneficial as it would allow the grazer to feed on more periphyton. Using a caddisfly grazer, Glossosoma larvae, we conducted a laboratory channel experiment with upstream experimental plates having four levels of periphyton abundance (control, low, medium, and high), and we recorded the movement behavior of the larvae. As periphyton abundance increased, both larval crawling time and the total crawling distance to the periphyton patch significantly decreased and the directness of the crawling path significantly increased. That is, larvae crawled in various directions as they approached a relatively low-abundance periphyton patch, whereas they crawled straight to the high-abundance periphyton patch in a short time. The behavior change was likely due to the detection of some microalgal cue, which should intensify with an increase in upstream periphyton abundance.

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